The invention relates to a process for preparing isocyanates. In one aspect, the invention relates to a new and useful process for preparing isocyanates from primary amines, carbon dioxide and an electrophilic or oxophilic dehydrating agent.
Isocyanates, especially diisocyanates, are important commodity chemicals for use in applications such as preparation of urethane foam, urethane elastomers, coatings, insecticides, herbicides, and the like.
Commercially, the phosgenation of primary amines is by far the most widely used method for producing isocyanates. The use of phosgene, however, has several disadvantages. The phosgenation route is long, energy intensive and requires handling highly corrosive materials, e.g. hydrogen chloride, chlorine, sulfuric acid and nitric acid, and highly toxic reagents and intermediates, e.g. phosgene and chlorine. Furthermore, the phosgenation route requires use of process equipment which can withstand high temperatures and highly corrosive conditions resulting in increased capital costs.
One non-phosgene method for the preparation of isocyanates involves reaction of primary amines and CO.sub.2 with a cobalt or manganese compound to produce metal carbamate complexes followed by reaction with an acyl halide in the presence of a solvent as is disclosed by A. Belforte et al., "Incorporation and Deoxygenation of Carbon Dioxide: A Metal-assisted Facile Conversion of Carbon Dioxide and Primary Amines To Isocyanates", Chem. Ber., 121, 1891-1897 (1988). However, the process described therein requires long reaction times and gives unsatisfactory yield of isocyanate for a commercially viable process.
Another non-phosgene route to isocyanates is found in U.S. Pat. No. 4,192,815 (Sheludyakov et al.) which discloses preparation of isocyanates by reacting a primary amine with CO.sub.2 and hexamethyldisilazane in the presence of an acidic catalyst, e.g. H.sub.2 SO.sub.4, followed by decomposition of the resulting silyl esters of carbamic acid in the presence of a dehydration agent. However, the process described therein requires long reaction times and is not commercially practicable.
A non-phosgene process for preparing isocyanates which is economical, commercially viable, and can produce isocyanates with high yield under mild reaction conditions and short reaction times is highly desirable.